When he was running for mayor last year, Marty Walsh pitched the 15 years he spent in the Legislature as an unmatched asset. Having friends in the State House would make him a rainmaker come budget time, he said, and the city’s unions wouldn’t dare cross him, because they’d fear his power to nuke them on Beacon Hill.

Since Walsh settled into City Hall in January, Bostonians have seen a different side of him. They’ve seen a guy who has spent his entire career hashing out issues in a room with a dozen colleagues, but who hasn’t appeared comfortable acting like Boston’s boss — at least in public.

That’s changing. Walsh is inserting himself into a dustup between state transportation officials and Allston residents. The battle for control of an Allston highway project presents the first time Walsh has grabbed the levers of mayoral power and asserted himself publicly. It shows the new mayor growing into the job, and seizing the chance to start building a legacy for himself.

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The state Department of Transportation is in the middle of an effort to demolish the Massachusetts Turnpike’s Allston toll plaza and rebuild the crumbling highway deck that runs between Boston University and the Charles River. As part of the $260 million project, the state plans to straighten the bend in the Allston Pike and rebuild the ramps that connect the highway to Allston and Cambridge.

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MassDOT’s decision to shift the Pike south could be the catalyst that’s needed to unlock the development of 140 acres of land Harvard owns around the Allston tollbooths. The land hasn’t been worth much up to now, because it’s been the site of a CSX freight rail yard, and because a tangled mess of highway ramps slice through the property.

By the time the highway reconstruction is complete, subway-like rail cars will be running through the Allston yard, shuttling between New Balance’s Boston Landing complex and South Station, and between Brighton and Kendall Square. This new transit network gives the formerly landlocked parcel enormous development potential. Neighborhood residents also hope the highway project will open up pedestrian and biking connections between Lower Allston and Commonwealth Avenue on one side and Cambridge on the other.

Walsh jumped into the Turnpike planning process last month because residents hoping to use the highway project to drive catalytic change have been sorely disappointed. MassDOT has thrown water on the movement to build a new transit hub, West Station, concurrently with the highway project. And Allston residents have repeatedly complained that MassDOT’s highway designers aren’t thinking about what happens to the site once road construction crews leave.

With neighborhood complaints mounting, Walsh pulled a coup. He sent a couple of aides to a recent meeting between MassDOT and Allston residents. The aides said they were concerned about where the highway project was heading; they said, with the residents’ blessing, that they would take charge of setting the vision for what the surrounding neighborhood will look like once the highway reconstruction is finished.

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City Hall is used to being the bad cop in Lower Allston, where it has paved the way for Harvard’s expansion. But in the Turnpike fight, Walsh’s administration has seized a populist role. The city’s chief planner, Kairos Shen, drew cheers two weeks ago when he said that nothing MassDOT had put on the table was acceptable to Walsh because “this seems all about the highway.”

“The mayor asked us to help bridge the communication divide,” Shen explains. “The mayor doesn’t want to slow the project down. He wants to keep the pace, but make sure all the issues that need to be on the table are on the table.”

Walsh is walking a delicate line in Allston. His aides go out of their way to praise MassDOT for tackling the highway project. But all the good planning in the world won’t matter if there’s still a highway running through the middle of the Allston rail yards. Walsh is also signaling that he sees the land around the Allston Pike as Boston’s next Seaport District. It’s where Boston’s next growth spurt will happen, and he’s going to work the levers of power to make sure the Pike reconstruction advances that vision, instead of stepping on it. It’s the first time Walsh has thrown his newfound weight around publicly. And, judging by the reaction in Allston, it’s a welcome development.

Paul McMorrow is an associate editor at Commonwealth Magazine. His column appears regularly in the Globe.